1. Field of the Invention
The present invention relates to a laser diode control circuit and a laser diode control method.
2. Related Background Art
An optical transmitter comprises a semiconductor laser and a laser driving circuit. In the field of optical communications, a semiconductor laser is driven by a laser driving circuit to generate signal light corresponding to a transmission signal.
The present inventors have performed research into the control of semiconductor lasers. In this research, the inventors investigated semiconductor laser control methods. As a result of this investigation, the following clarifications were made.
In a first technique, the optical output power and extinction ratio of a semiconductor laser are controlled by detecting the average value and peak value of a monitoring photodetector. However, according to the investigations of the inventors, it was learned that when the optical signal transmission rate increases, it becomes difficult to operate the monitoring photodetector at a high enough speed to detect the peak value in the received optical signal.
In a second technique, a control circuit converts the output of the monitoring photodetector into a digital value using an A/D converter and subtracts this digital value from a reference value stored in memory. The control circuit converts the subtracted value into an analog value using a D/A converter and controls the semiconductor laser on the basis of this analog value. In this controlling technique, an optimum modulation current value for each temperature is stored in memory and the extinction ratio is held at a constant level on the basis of this value. In this technique, for the purposes of control, the modulation current value is determined uniquely using a signal from a thermistor for detecting ambient temperature, and the bias current is determined such that the average power is maintained at a constant level. However, it was learned from the investigations of the inventors that in this technique, when the luminous efficiency decreases due to deterioration in the laser light generating portion over time, only the bias current is increased. Thus the extinction ratio cannot be maintained at a constant level.
In a third technique, the bias current, the modulation current, and an initial value for the monitoring photodetector in respect of the usage range temperature are stored in memory, and the modulation current and bias current are determined using a signal based on the result of a comparison between a signal from the monitoring photodetector and the values stored in the memory. In this technique, the modulation current and bias current are determined using a signal from a temperature sensor. However, according to the investigations of the inventors, it is impossible with this technique to compensate for changes due to deterioration of the semiconductor laser over time.
According to these investigations, it was discovered that technical problems exist in conventional techniques such that a technique for controlling the optical output power of a semiconductor laser toward a predetermined value and a technique for controlling the extinction ratio of a semiconductor laser toward a predetermined value have not been achieved.
It is therefore an object of the present invention to provide a laser diode control circuit and laser diode control method for performing control of the optical output power and extinction ratio of a laser light generating portion having a semiconductor light source.
A first aspect of the present invention relates to a laser diode control circuit. The laser diode control circuit comprises a load portion, a control portion, a bias current circuit portion, and a modulation current circuit portion. The load portion generates a signal corresponding to a photocurrent which is generated by a photodetector in accordance with the optical power of the light received from a laser light generating portion. The control portion generates a first control signal for modifying one of a bias current and a modulation current and a second control signal for modifying the other of the bias current and modulation current from among a value X, a value Y, an amount of change xcex94X in relation to the value X, and an amount of change xcex94Y in relation to the value Y. The first control signal is generated from xcex94Db and a value corresponding to Db. The second control signal is generated from xcex94Dm and a value corresponding to Dm. The amount of change xcex94Y is generated by a function f(X, xcex94X)=xcex94Y which is defined such that the light emission power displays a predetermined dependence and the extinction ratio displays a predetermined dependence. A set of variables (X, Y, xcex94X, xcex94Y) is either (Db, Dm, xcex94Db, xcex94Dm) or (Dm, Db, xcex94Dm, xcex94Db), the symbol Db indicating a value corresponding to a bias current Ib, the symbol Dm indicating a value corresponding to a modulation current Im, the symbol xcex94Db indicating a value corresponding to an amount of change xcex94Ib in the bias current Ib, and the symbol xcex94Dm indicating a value corresponding to an amount of change xcex94Im in the modulation current Im. The bias current circuit portion is connected to the laser light generating portion and generates a bias current in accordance with the first control signal. The modulation current circuit portion is connected to the laser light generating portion and generates a modulation current in accordance with the second control signal.
In order to control the bias current and modulation current in this control circuit, the amount of change xcex94Y is generated by a function f(X, xcex94X)=xcex94Y in relation to a set of variables (X, Y, xcex94X, xcex94Y)=(Db, Dm, xcex94Db, xcex94Dm) or (X, Y, xcex94X, xcex94Y)=(Dm, Db, xcex94Dm, xcex94Db), and hence a control circuit which is capable of control such that the light emission power displays a predetermined dependence and the extinction ratio displays a predetermined dependence is provided.
In the laser diode control circuit, the function f may be constituted as an approximated function of a function defined such that the light emission power becomes constant and the extinction ratio becomes constant.
In the laser diode control circuit, the control portion may be constituted so as to comprise first means, second means, and third means. The first means generates the amount of change xcex94Y in accordance with the function f(X, xcex94X)=xcex94Y and in accordance with the amount of change xcex94X which is determined according to the result of a comparison between a value corresponding to the signal which corresponds to the photocurrent and a reference value. The second means generates the first control signal from values corresponding to xcex94Db and Db. The third means generates the second control signal from values corresponding to xcex94Dm and Dm.
In the laser diode control circuit, the first means may comprise nonvolatile memory. The laser diode control circuit may further comprise an interface portion for communicating with an external device and means connected to the interface portion for performing a write operation to the nonvolatile memory. According to this constitution, the laser diode control circuit can be adjusted via the interface portion. For example, by providing the interface portion, the amount of labor required for each individual adjustment operation in the laser diode driving circuit can be reduced in comparison with manual adjustments. Further, when the characteristic data for each laser diode are stored in an external device which is a computer, adjustments can be performed in alignment with the characteristic data.
In the laser diode control circuit, first storage means may store a constant (xcex94Y/xcex94X)o in relation to the entire range of the variable X. This constitution is easily realized and allows a reduction in the circuit scale and the number of processing steps.
In the laser diode control circuit, the first storage means may store a constant (xcex94Y/xcex94X)n in association with each of a plurality of regions (Rn: nxe2x89xa71) into which the entire range of the variable X is divided. This constitution is comparatively easy to realize and also allows the light emission power and extinction ratio to be held to predetermined characteristics in a wide temperature range regardless of using a simple approximated function.
In the laser diode control circuit, the first means may comprise first generating means, comparing means, second generating means, and third generating means. The first generating means generates a value (xcex94Y/xcex94X)m which is calculated from a linear function f in relation to X in an arbitrary region Rm from among the plurality of regions (Rm: mxe2x89xa71) into which the entire range of the variable X is divided. The comparing means compares a monitoring signal corresponding to the photocurrent value with a reference value. The second generating means determines the value xcex94Y on the basis of the value of (xcex94Y/xcex94X)m and on the basis of a value of xcex94X which is set such that the optical power of the light increases when the comparison result in the comparing means indicates that the optical power of the light is smaller than a predetermined value. The third generating means determines the value xcex94Y on the basis of the value of (xcex94Y/xcex94X)m and on the basis of a value of xcex94X which is set such that the optical power of the light decreases when the comparison result indicates that the optical power of the light is larger than a predetermined value. This constitution is comparatively easy to realize and allows the light emission power and extinction ratio to be held to predetermined characteristics in a wide temperature range with a high degree of precision.
In the laser diode control circuit, the control portion may comprise means for comparing any one of Db, Dm, or the sum SUM of Db and Dm with a threshold, and generating a first comparison signal indicating the result of this comparison, and means for generating a first alarm signal indicating the presence of an overcurrent in the laser light generating portion when this first comparison signal indicates that the current is excessive high. Thus deterioration of and breakdowns in the laser light generating portion can be detected. The control portion may also comprise means for halting the first alarm signal on condition that the first comparison signal indicates a normal current when the first alarm signal is being asserted.
In the laser diode control circuit, the control portion may comprise means for comparing a monitoring signal which corresponds to the photocurrent with a threshold, and generating a third comparison signal which indicates the result of this comparison, and means for generating a second alarm signal indicating a decrease in light emission power in the laser light generating portion when the third comparison signal indicates excessively low power. Thus deterioration of and breakdowns in the laser light generating portion can be detected. The control portion may also comprise means for halting the second alarm signal on condition that the third comparison signal indicates normal power when the second alarm signal is being asserted.
By using both the first comparison signal and the third comparison signal, a state of deterioration in which a predetermined optical power is generated but the power supply is excessively large may be differentiated from a state of breakdown in which light is not emitted, for example. Thus, unforeseen breakdowns of the system can be avoided and as a result the optical module can be replaced in advance, prior to a breakdown.
The laser diode control circuit may further comprise an interface portion for communicating with an external device, means for halting the generation of light in the laser light generating portion in response to a signal from the interface portion, and means for commencing the generation of light in the laser light generating portion in response to a signal from the interface portion. According to these means, the output of light can be halted when disconnection of an optical connector is detected, and thus the danger of laser light leakage can be avoided. Further, since the output of light can be commenced and halted intentionally, transmission in an optical fiber, for example, can be ensured.
Another aspect of the present invention relates to a laser diode control method. This method comprises the steps of: (a) generating a signal which corresponds to a photocurrent from a monitoring photodetector for receiving light from a laser light generating portion; (b) generating a first control signal using xcex94X and X for controlling one of the bias current and the modulation current and generating a second control signal using xcex94Y and Y for controlling the other of the bias current and the modulation current from among a value X, a value Y, a variable xcex94X which is defined in accordance with the result of a comparison between a value corresponding to this signal and a reference value, and a value xcex94Y which is generated by a function f(X, xcex94X)=xcex94Y defined such that the light emission power displays a predetermined dependence and the extinction ratio displays a predetermined dependence; and (c) driving the laser light generating portion using the bias current and modulation current generated on the basis of the first and second control signals. A set of variables (X, Y, xcex94X, xcex94Y) is one of either (Db, Dm, xcex94Db, xcex94Dm) or (Dm, Db, xcex94Dm, xcex94Db). The symbol Db indicates a value corresponding to a bias current Ib, the symbol Dm indicates a value corresponding to a modulation current Im, the symbol xcex94Db indicates a value corresponding to an amount of change xcex94Ib in the bias current, and the symbol xcex94Dm indicates a value corresponding to an amount of change xcex94Im in the modulation current.
In this laser diode control method, the generating step (b) may comprise the steps of: (d) comparing the absolute value of the difference between the value corresponding to the signal and the reference value with a predetermined value; (e) when this absolute value exceeds the predetermined value, generating the first control signal using xcex94X and X for controlling one of the bias current and the modulation current and generating the second control signal using xcex94Y and Y for controlling the other of the bias current and the modulation current wherein the value xcex94X is defined in accordance with the result of a comparison between the value corresponding to the signal and the reference value, the value xcex94Y is generated by the function f(X,xcex94X)=xcex94Y defined such that the light emission power displays a predetermined dependence and the extinction ratio displays a predetermined dependence; and (f) when this absolute value is equal to or less than the predetermined value, maintaining the first and second control signals. According to this method, wavering of the bias current and modulation current can be reduced.
According to these inventions, a laser diode control circuit and a laser diode control method for performing control of the optical output power and extinction ratio of a laser light generating portion comprised in a semiconductor light source are provided.
The aforementioned object and other objects, features, and advantages of the present invention will become clearer from the detailed description in the following preferred embodiments of the present invention which will progress with reference to the attached drawings.
However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.